By default, Mailspring displays a notification whenever one of your emails is opened or a link is clicked. If a recipient opens the email several times throughout the day, or uses a client that displays the message frequently as they click through their mailbox, you may get many events.

If you'd rather not be notified about repeated reads and link clicks, visit Mailspring's preferences and uncheck the "Show notifications for repeated opens / clicks" option in Preferences > General: